PITA : Hub Builds On Fat Bike Rims 101

I’ve been procrastinating about doing this post for a while now because I honestly feel like hub motors have no place at all on trail-riding fat-bikes except maybe as the front wheel of an AWD ebike. The wheels are way too big and the torque is far to limited to be of any use at all in the snow. That being said I’ve done about 10 wheel builds now (mostly disasters) and they have all been total nightmares. Hopefully I can help other people have a little less frustration and waste a lot less time than I did to get it right.

This front wheel build took several tries to get right.

First I tried to buy pre-laced motors direct from China. Then I got killed on shipping ($300+ in shipping alone to get 2 built up rims with $80 motors on them). Then I took them apart thinking that I could just swap out their crappy rims to fat 80mm rims and get to use the same spokes. Wrong! Then I went looking for longer nipples but the spokes that I got with the wheel builds were 2.2 mm which are totally impossible to find in the US. Eventually I ended up blowing more money on spokes that were also the wrong size and was shamed time and time again when I brought my wheel set to the bike shop for their professional critique. What an amateur am I.

It seems like no matter what spoke length calculator I use, whatever spoke length it tells me to use is always off by at least 3-5mm. You can get away with having spokes that are a couple of mm too short by buying longer nipples 16mm used for Aero rims. The best spoke calculator I’ve found yet is Justin’s here, but even that one tends to give me the wrong spoke sizes. I would advise erring on the side of too short and buy the extra long 16mm nipples to take up the slack if you must. It takes me an average of 2.5 builds to get any wheel build right. I can mount a BBS02 start to finish in about 2 hours but I would say that on average I spent at least 6 hours wasted on every hub lacing I do.

80mm wide and comes in 32 or 36 hole for about $50. Origin 8 Dat-Pro-80

There are a lot of problems mounting hub motors on fat bike rims. The first problem you’re going to have is almost every motor on the market is 36 hole, but almost every fatbike rim on the planet is 32 hole. The rims I’ve settled on is the hefty Origin 8 DAT-PRO-80 doublewall rim which I get on fleabay for about $50 each shipped (make sure it’s 36 hole not 32). These seem to mate properly with the Mac or Fat Mac motor with 208mm spokes if you’re not doing any offsetting. If you do need to offset the rim then all bets are off and your best bet is to cross the spokes over to the opposite side, this is the only way I know of to get the rim to offset more than a few mm. Some fatbike frames have the rear rim massively offset to one side or the other so the chain doesn’t hit the tire.

The crushed washers will practically disappear when you tighten the spokes up.

Be aware than when you use 14 Gauge spokes on most ebike motors (which are really designed for thicker 13 Gauge spokes) you should really use tiny Stainless washers on the spokes. The washers will deform and keep the spoke from breaking right at the elbow where it will rest against the motor if there is no washer. I use M2 stainless washers which can bought from fleabay about $3.50/qty100 shipped.

The other thing I do is forget about the truing stand and just center the wheel by sound. If you tighten all the nipples the same amount, then for the final tightening make sure that all the spokes twang at about the same tone then your wheel build should be very close to true every time without the need for a truing stand. You can always throw the rim on the bike and just use the front forks or chainstay as a poor-mans truing stand with a chopped off ziptie as the guide. I do it all the time (read ‘too cheap to take his wife or kid to the movies’). Many fancy truing stands won’t even fit a fatmac with a 100mm rim.

Crossing the spokes to the opposite side is the only way to offset the rim. Man, that rim is FILTHY! Am I really going to get advice from someone who can’t even keep their rims clean?

If I haven’t managed to talk you out of doing a hub lacing for a fat bike build let me at least offer these suggestions.

When you buy spokes, get them 2mm less than the spoke calc software tells you too. It seems that my spokes are always too long on the first build. If they don’t fit then get the 16mm nipples (brass only no aluminum ever) which have much more thread and margin for error.

I recommend doing a single cross for bigger motors (like the mac) and double cross for smaller 5lb and less motors.

If the spokes are still too long after your first build (meaning they stick out of the nipple end more than 1mm) then re-thread it with the spokes crossed over to the opposite side. This will eat up 2-3 mm and make for a much stronger build.

If you need to offset the motor to one side or the other more than a mm or two then you MUST cross over to the opposite side and it is likely that the spokes will have wildly different lengths.

When offsetting a wheel one side will have much higher tension in the spokes than the other will.

I’ve had good luck getting the longer aero nipples and small packages of spokes from biketoolsetc.com. Em3ev also sells custom cut spokes to any length for $.55 each (including nipple) and they are very high quality 13 gauge sapim spokes found here (settings for Mac motors on this page as well) which can be hard to find. Paul suggests using the sapim calculator here which is missing some key features like inputting crossover to the opposite side, dishing or spoke hole offsets on the rim which most fatbike rims have. If you use it be aware that your spoke length it quotes you will usually be a few mm too long.

There are a lot of useful stuff on the internet for doing wheel builds. I used a bunch of youtube videos and this link here by Sheldon Brown the Alpha Bike Nerd of the Universe. Our friends at http://www.electricbike.com have written a fantastic article about lacing hub motors that I have yet to read here. You can easily waste countless hours of your precious life learning how to build wheelsets around hubmotors or you can do the smart thing and invest in a decent mid-drive unit.

If you’re somehow good at maths or physics it isn’t a big deal to calculate the required spoke length yourself. Or ask a friend wo claims to be good at such things. Of course the measurements need to be as precise as possible.